1. Field of the Invention
The present invention relates generally to fuel supply systems, and particularly to a vehicle engine fuel supply system, having a sub-tank in which a fuel pump is installed.
2. Related Art
JP-A-2-30971 discloses a fuel supply system for a vehicle engine having a fuel tank, a sub-tank in which a fuel pump is installed and a jet pump having an injection hole. In the fuel supply system, a fuel level in the sub-tank is maintained so that the fuel pump sufficiently sucks fuel from the sub-tank even when a fuel level in the fuel tank is lowered. Fuel returned from the engine is supplied to the jet pump which injects fuel into the sub-tank through the injection hole. When fuel is injected into the sub-tank, a negative pressure is generated around the injected fuel, and fuel in the fuel tank is also sucked into the sub-tank due to the negative pressure. Thus, fuel is constantly supplied to the sub-tank so that the fuel pump installed therein constantly sucks fuel from the sub-tank.
The sub-tank has a fuel inlet passage through which fuel injected by the jet pump and fuel from the fuel tank is introduced. A water head pressure corresponding to the fuel level in the sub-tank is constantly applied to the fuel inlet passage. When an amount of fuel remaining in the fuel tank is relatively large, fuel exists in a space around the injection hole of the jet pump in the fuel tank. Therefore, when fuel is injected into the fuel inlet passage by the jet pump, fuel is sucked into the fuel inlet passage due to the negative pressure and disperses over the entire cross-section of the fuel inlet passage along with the injected fuel. As a result, fuel in the sub-tank is prevented from being discharged through the fuel inlet passage toward the jet pump, and fuel in the fuel tank is introduced into the sub-tank in opposition to the water head pressure of the fuel in the sub-tank.
However, when the amount of fuel remaining in the fuel tank is decreased, and the vehicle is steered around a sharp curve or is driven on a road having a steep grade, fuel may not be dispose around the injection hole of the jet pump in the fuel tank. In this case, only fuel injected by the jet pump opposes the water head pressure of the fuel in the sub-tank. If fuel injected by the jet pump disperses insufficiently and a fuel membrane is not formed over the entire cross-section of the fuel inlet passage, fuel in the sub-tank may be discharged through a gap created between the fuel membrane and an inner wall of the fuel inlet passage. Further, if fuel is not continuously present in the space around the injection hole of the jet pump the amount of fuel in the sub-tank will greatly decrease.
Further, in the fuel supply system, a diameter of the fuel inlet passage is set relatively large (e.g., 10 mm) so that fuel in the fuel tank is spontaneously introduced into the sub-tank through the fuel inlet passage when fuel initially fills the fuel tank. When a fuel level in the fuel tank is higher than the fuel inlet passage, a gap between the injected fuel and an inner wall of the fuel inlet passage is filled with fuel. In this case, fuel is prevented from being discharged from the sub-tank although the injected fuel does not hit the inner wall of the fuel inlet passage.
Since an injection angle of fuel injected by the jet pump is relatively small, the injected fuel never contacts the inner wall of the fuel inlet passage. Therefore, when a fuel level in the fuel tank is as low as a bottom surface of the fuel tank and no fuel exists around the fuel inlet passage, a gap is created between the injected fuel and the inner wall of the fuel inlet passage. As a result, fuel in the sub-tank is discharged through the gap. Conventionally this only happens when the amount of fuel remaining in the fuel tank is extremely small. However, recently, a vehicle capable of being steered around with a maximum load with a small amount of fuel remaining in the fuel tank is in great demand. In such a vehicle, it is necessary to maintain the fuel level in the sub-tank even if the fuel level in the fuel tank is low.